Researchers at MIT have developed optical fibers that not only carry and modulate light, but also generate and sense pressure changes. The multifunctional fibers could be used to make various types of sensors. The fibers can also be squeezed in a way that modulates an optical signal, making them promising for “smart” textiles.
“We want to increase the level of complexity and sophistication of fibers,” says Yoel Fink, professor of materials science and engineering at MIT.
By integrating heat- and light-sensitive materials during the manufacture of optical fibers, Fink’s group has previously made fibers that act as simple sensors and even cameras. They’ve now added a new level of functionality to optical fibers by introducing a layer of piezoelectric material. This material converts electrical signals into a mechanical change, and vice versa, meaning pressure can be applied, or sensed, in the fiber.
The main challenge in making these fibers is in precisely arranging layers of multiple materials and processing them under conditions that lead to quality layers. Over the past several years, Fink’s group has developed a process for carefully layering materials to form a thick “preform” rod that is heated and stretched to make a very thin, kilometers-long fiber that contains different materials, including polymers and metals.
The key to the approach is selecting materials that not only have the desired properties but that also melt and flow at the same temperature. For the piezoelectric fibers, Fink makes a preform that is 40 millimeters in diameter. It contains a polymer that forms a high-quality piezoelectric crystal as it cools down, and a polycarbonate material that is both viscous and conductive. When heated and stretched, the dimensions of these layers shrink from millimeters to nanometers, while maintaining the same ratio of thicknesses. The new fibers could be especially useful for distributed sensing and imaging because they’re so thin, flexible, and lightweight.
“The big challenge in integrating functionality is integrating very different materials, and this is a big step forward,” says Ritesh Agarwal, professor of materials science and engineering at the University of Pennsylvania. Agarwal says it’s impressive that the piezoelectric layer retains its properties after being stretched out–the MIT researchers have developed manufacturing conditions that ensure that the crystalline structure of this material, which is important to retaining its pressure-to-electricity converting properties, is maintained.
The finished fiber has a core that can carry light, a piezoelectric layer, and electrodes that can carry electricity to and from the piezoelectric layer. The MIT researchers can send pulses of electrical current down the fiber, causing the piezoelectric layer to squeeze the fiber. The resulting vibrations can be used to create acoustic waves, and the fibers can also detect vibrations and changes in pressure, because these, in turn, generate an electrical signal. This work is described this week in the journal Nature Materials.
Fink believes there are many possible applications for the new fibers. They could be woven into carpets that can count the number of people walking across them, or integrated into structural composites and used to sense cracks before they become serious. But one of the most promising applications, Fink believes, is in biomedicine. The fibers are less than a micrometer wide–narrow enough to be snaked into blood vessels or inserted into organs to monitor heart rate, blood flow, or biomarkers in the blood. Their ability to carry infrared light and to perform acoustic sensing offers a combination of properties similar to an ultrasound imager, a heart-rate monitor, and chemical spectrometer.
“Having the piezoelectric and the optical fiber completely integrated makes the sensor much smaller,” says Juan Hinestroza, professor of fiber science and leader of the Textiles Nanotechnology Laboratory at Cornell University. “This is important–especially in a blood vessel or in a composite material where you have very little room.”
The piezoelectric layer of the MIT fiber can be used to modulate the optical signals bouncing off the insides of the fiber. Fink’s group has also made fibers containing a reflective layer that act as a sort of optical switch. The reflective layer interacts with specific wavelengths of light, determined by the thickness of the layers. Running an electrical pulse through the fiber squeezes the mirror, changing the color of light with which it will interact. If woven into a fabric the fiber could produce different visible patterns of color. “If you wanted to read information off a piece of clothing, or a plane or car, you could integrate these fibers,” says Fink.
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